*CB TO SIX METRES* Part1 - The Introduction
By Shawn Baris ZR1EV
PO BOX 212
Brackenfell
7561
Republic of South Africa
*Anyone is free to use the following information for private use on the
provision that it is not used for commercial purposes . Permission is
granted by the author , ZR1EV , to publish and or distribute all or part of
the following on the condition that recognition is given to the author .In
the case of it being used in a newsletter or magazine , a copy of the
aforementioned should be sent to the above adress.*
Having allways had an interest in "band 1" propagation , and being spurred
by reports of TV signals from the old Crystal Palace transmitter on or
around 45Mc being recieved regularly down here in Cape Town during the
Seventies , It was particularly frustrating not to be able to operate on
the 50Mc band . This band was for exclusive use by ZS-call holders and as
you can see by my call , I hold a "VHF-only , no-code" ZR-call .
Things changed in 1981 , when it was announced that the six metre band
will be opened up for holders of the ZR-call on the 1st of January 1982 . I
was ecstatic! But elation was soon followed by disapointment when it became
apparent that there were very few rigs to be had , and the prices of those
available were way above what I could afford .
I was given an ex-military rig by OM Dave , ZS1SG , but soon realised that
if I was to really exploit the amazing fare of different propagation modes
possible on six metres , I would have to use SSB/CW and have more power
at hand , when it was needed . A transverter seemed to be the obvious
choice at the time , and I proceeded to aquire an SSB/AM CB rig , with the
idea to convert it for operation on the 10 metre band , which I would then
use to drive a transverter I was still going to construct .
After the mod to the rig , (which worked extremely well) , I proceeded to
experiment to find the best circuit for a transmit converter that would
eventually form part of my prototype . At this point , I realised that the
whole idea is not going to be all that simple . The local oscillator would
have to have a frequency of 22 Mc . (22 + 28 = 50) . Unwanted products from
the mixer would be (28 x 2 = 56) and (22 x 2 = 44) These unwanted products
from the mixer i.e. 56- and 44 Mc are only 6 Mc away from the wanted output
and were going to be very difficult to attenuate to the required -50dB
(min) level required and to have a consistant clean output across 50-52 Mc
would be asking quite a lot from the mixer and filter stages . To cover
from 50-54 would paint an even bleaker picture and would involve switching
bandpass filters to enable the unit to stay within minimum spec .
At this point , something else dawned on me . For the rig to operate on 28
Mc , the local oscillator would have to be on (28.000 + 10.695 = 38.695) .
The 10.695 is the IF frequency used on the rig I had and the local
oscillator is injected on the "high side" of the desired operating
frequency . It now becomes clear that if the Local oscillator signal could
be treated as if it would be injecting on the "low side" of the desired
operating frequency , the new frequency (image of 28.000 , with 10.695 IF)
would be 38.695 (LO) + 10.695(IF) = 49.390!
If the Local oscillator signal could now be made to go up to 39.305 (rig
operating on 28.610) we now have (39.305 + 10.695 = 50.000!) . This forms
the basis of the modifications to follow . In part2 , I will discuss the
frequency synthesizer , and how to actually go about moving the rig up to
round 28.600 , which will be the first step in the mod . Also , bypassing
the inherent problems when using a 40 CB channel switch in any mod , albeit
to 10M or to 6M . (5 "gaps" where the frequency "jumps" 20kHz and the
sequencing problems round ch23-25 , and the nuisance of having to have a
typed piece of paper stuck to the rig to tell you what channel=what
frequency) The conversion is well worth the effort and I have already done
dozens of these mods . The bugs in the early prototypes have been fixed and
due to pressure from the local guys , I have decided to "put pen to paper"
and share what I found from years of experimenting and perfecting .
Hope you will be able to enjoy it as much as I have . Please let me have
your comments and input .
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*CB TO SIX METRES* Part2 - The PLL Frequency Synthesizer
By Shawn Baris ZR1EV
PO BOX 212
Brackenfell
7561
Republic of South Africa
*Anyone is free to use the following information for private use on the
provision that it is not used for commercial purposes . Permission is
granted by the author , ZR1EV , to publish and or distribute all or part of
the following on the condition that recognition is given to the author . In
the case of it being used in a newsletter or magazine , a copy of the
aforementioned should be sent to the above adress.*
For the purposes of this discussion , I will assume the following :(1)That
you have a working knowlege of RF circuitry and techniques .(2)That you
have the essential tools to do such a conversion . (3) Basic RF test
equipment .(4) The set you plan to convert is fully functional .(5)The set
is already fitted with a 40-channel selector switch .(6)That you have the
service / workshop manual of your rig .
The chassis I will be discussing is the UNIDEN PTBM048AOX -chassis fitted
to a number of CB rigs parading under different brand names . It is a
single conversion radio on SSB and double conversion on AM , based on the
popular PLL02A PLL chip , and has the Mitsubishi 2SC2166 and 2SC1969 in the
TX lineup . Some versions of this rig operated in the "C" band (29MHz
Marine) and had a diode matrix , six position channel switch , and could be
set to step in 12.5 kHz increments . In addition , these units also had a
"piggy-back" pcb fitted near the VCO , containing a 12.800 MHz oscillator .
These sets should be changed back to the original 27 Mhz configuration and
tested before any modifications are attempted .
The PLL02A(G) chip is quite remarkable in that it contains practically all
the components needed for a PLL synthesizer , exept the VCO , of course .
The chip sports a 9-bit binary divide-by-N-counter , a phase comparator
with lock detect o/p (pin6) , and a fixed divider of either "divide-by-1024
or divide-by-2048" , selectable at pin 4 . In this application , a 10.240
Mc oscillator feeds into this fixed divider at pin3 , and when pin4 is left
o/c (logic1), will divide this signal by 1024 , resulting in 10kc , which
will be your synthesizer "step" . (If pin4 is tied to ground -logic0- the
fixed divider will now divide by 2048 , giving you a 5kc "step") DO NOT
change the 10.240 frequency . The output of that oscillator is also used in
the AM reciever section for the conversion of the 10.695 IF down to 455kc .
(10.695 - 10.240 = 0.455Mc or 455kc) !
At first glance , it seems as if the chip might be able to cover over
5MHz in one go (512 binary combinations at 10kc cannel spacing),but when
one has a good look at the spec sheet , the maximum frequency allowed into
the divide-by-N counter is limited to 3.5 MHz . So , at 10kc channel
spacing , the divide-N binary number of about 350 should not be exceeded to
ensure that the chip runs within it's specs . This is the main reason that
the mixer xtal (X1) 10.0525 MHz has to be changed .
IC2 functions as both the VCO and as a mixer (C3001/TA7310P) . The
interesting thing about this mixer is that it has two inputs (f1 and f2) ,
and two outputs (f1+f2 and f1-f2) . The difference output is taken off pin9
and the sum output is taken from pin6 . The difference output is fed into
the divide-by-N counter input , and the sum output is filtered and is used
as the Local Oscillator signal . X1 (10.0525) and Q3 make up the oscillator
and doubler stage .
As an example , channel 19 (27.185) , the VCO will run at 17.775 , mixed
by the output of the doubler 20.105 by IC2 and will produce two outputs :
the sum output, 37.880 and the difference output , 2.330 . The 2.330 goes
into the divide-by-N counter , where it is divided according to the value
in binary applied to pin7 (MSD) to pin15 (LSD) , in this case a value of
divide by 233 . The result of this division is 10kc , where it gets
"compared" to the 10kc "reference" frequency from the divide-by-1024 fixed
divider chain by the Phase comparator . The output from this "charge pump"
is fed to a LPF where the "error" is filtered to DC , so that it can be
used to either force the VCO frequency up or down untill there is no error.
The loop is now "locked" and pin6 will go to logic1 (5.4V) . The PLL (Phase
Locked Loop) .
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*CB TO SIX METRES* Part3 - The PLL Frequency Synthesizer continued
By Shawn Baris ZR1EV
PO BOX 212
Brackenfell
7561
Republic of South Africa
*Anyone is free to use the following information for private use on the
provision that it is not used for commercial purposes . Permission is
granted by the author , ZR1EV , to publish and or distribute all or part of
the following on the condition that recognition is given to the author . In
the case of it being used in a newsletter or magazine , a copy of the
aforementioned should be sent to the above adress*
In part2 , we had a brief look at the basic operation of the PLL frequency
synthesizer (If you want a more detailed description , do drop me a line)
and from that we can see how the channel frequency is generated . If now ,
for example , we change the binary input to the PLL chip to something else
, say 234 , it would result in the phase detector generating an "error" ,
which will drive the VCO to the new frequency so that the loop will once
again be in the locked state . As the VCO will now have moved 10kc , the
resulting sum output will also have incremented by te same amount ,
resulting in the transceiver effectively having changed frequency by 10kc .
This scheme is fine for changeing frequency round the design operating
frequency of the radio , but to change "band" to a frequency more than 1MHz
up , one has to watch out not to exceed the 3.5MHz maximum frequency input
to the divide-by-N counter . It should be clear that the synthesizer
frequency can be moved higher by changeing the 10.0525 Xtal to a new value.
There should be no need to rush out and buy one , just have a look in your
"junkbox" for a Xtal with a fundamental frequency round 10.400 . After you
have done the replacement the alignment is easy . Attach an RF voltmeter /
'scope / RF "sniffer" to test point3 (TP3) and , using the propper (!)
trimming tool , adjust the core of T3 for max deflection . Then , connect a
DC voltmeter to pin6 of the PLL02A chip (take care not to short any of the
pins) and carefully adjust the core of the VCO "block" until you get a
solid 5V in that pin . Flick the channel switch from CH1-CH40 while
checking the voltage on pin6 of the PLL ic and confirm that it stays 5V .
The PLL is now locked . Connect the RF Voltmeter/'scope/sniffer to pin4 of
IC3 (C3001/TA7310P) and adjust T1 and T2 for maximum deflection on
Channel20 . Remember that the rig will not transmit if the PLL is not
locked . When pin6 of the PLL chip falls below 5V , it "switches off" Q7 in
the transmitter predriver stage , rendering the transmitter inoperative .
Set the rig in the transmit mode and realign the set as per the manual for
maximum output . Rig should now be transmitting somewhere around 28.600 and
the exact frequency can be checked with a frequency counter .(Please
remember to have the rig in the AM mode for measuring the frequency -
sounds obvious , but easy to forget in the heat of the moment)
You will now most probably be on a somewhat "wierd" frequency like 28.7842
or suchlike . (At no time touch the 10.692MHz oscillator trimming caps ,
CT4 and CT5 - I will explain their function later . They should not be
touched at all if your rig was operating 100 percent on 27Mc) . The easiest
way to go about the following procedure , is to have an HF reciever on hand
with SSB fascilities . In the previous example , say 28.7842 , tune your
reciever to either 28.780 or 28.790(preffered) . Key up the rig again (in
AM mode) and adjust CT1 for a zero beat on your reciever (Which should be
set to USB) The rig should be keyed into a "Dummy load".
Now set your reciever to LSB and do likewise on your rig and while
modulating , adjust CT2 for a natural sounding voice on your reciever . If
necissary, experiment with the values of C20 and C21 to get the rig spot on
frequency . You should get no less than 4W on AM and 12W on SSB after the
mod , and if you want , you can realign the reciever strip now as well
using an adjustable signal generator and use the set as a 10M rig , or get
ready to continue the mod up to six metres . Good luck .
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*CB TO SIX METRES* Part4 - The Reciever Section
By Shawn Baris ZR1EV
PO BOX 212
Brackenfell
7561
Republic of South Africa
*Anyone is free to use the following information for private use on the
provision that it is not used for commercial purposes . Permission is
granted by the author , ZR1EV , to publish and or distribute all or part of
the following on the condition that recognition is given to the author . In
the case of it being used in a newsletter or magazine , a copy of the
aforementioned should be sent to the above adress.*
Now that the rig has been converted to 10 meters and it has been
thoroughly tested there , we can proceed with the conversion to 50Mc . You
may by now have noticed that the chassis of the rig is isolated from the
earth-plane on the printed circuit board itself . This was done to enable
the rig to be installed in either a positive- or negative earth vehicle .
The chassis must be at RF earth though , as the antenna socket must "see"
the earth on the PCB . This is where a lot of "conversions" fall short in
performance . So what needs to be done first is to replace all the disk
ceramic capacitors that connect earth and ground together . All round the
PCB where it is screwed to the metal chassis , you will see them . Don't
forget the two caps on the DC connector ! Change all the capacitors with a
value of 0.047æF (47nF) to 0.022æF (22nF) .
Now for the reciever section . Change C100 (33pF) to 18pF and remove T7 .
Carefully disassemble T7 so you can get to the windings . Hold the coil
former upright (pins facing down and the side with the 3 pins facing you) .
CAREFULLY liberate the wire from the righthand pin and unwind the existing
windings , taking care not to break the delicate strand . You'll see that
the former has 4 "ribs" to hold the winding . Leave the wire attached to
the leftmost pin and proceed to first wind 4 turns onto the second "rib"
from the top in a clockwise direction . The "ribs" have a gap in the middle
so cross over to the topmost "rib" and wind 4 more turns on there in the
same direction , feed the wire down to the righthand pin and solder it onto
the pin . The secondary winding stays as is . Carefully reassemble , not
forgetting the green plastic spacer on top of the coil former , and solder
back into the printed circuit board . T7 is a Mitsumi type ETR0333 .
Next , change C102 (47nF) to 22nF disk ceramic . Remove T8 and dismantle
as with T7 and remove the small tubular ceramic cap . I found that a small
surgical knife to be ideal here , as you can then just cut the leads of the
capacitor , instead of battling to get them off the pins in an orderly
fashion . You will now notice that this time the winding has a tap that
goes to the centre pin . In this rig the tap is not used , so we can just
rewind as for T7, except this time it will be 1turn second rib from the top
and 4turns on the topmost rib back to the righthand pin and solder .
Assemble and solder back into the printed circuit board . This time you
have to add a capacitor to the print side of the board . T8 has 5 pins ,
grouped 2 on the one side and 3 on the other : the side with the 3 pins is
the primary in this case . Solder a 39pF ceramic capacitor accross the
outermost 2 pins of the primary winding on the foil side of the board . T8
is a Mitsumi type 10CA006 .
Now remove T9 and strip down as before . This coil has a pink pot core and
is a Mitsumi type 10CB001 . Wind 3turns onto the second rib from the top
and 1turn on the topmost rib . Solder back into the printed circuit board
and add a 39pF capacitor onto the primary as for T8 .
If your rig was operating for eg. on 28.790 on a specific channel , switch
it back to that channel and calculate the new six metre frequency as
follows : [10metre frequency] + 21.39 = [six metre frequency] . So in our
example it would be : 28.79 + 21.39 = 50.180 . Set your RF generator to
that frequency and (using the correct trimming tool) adjust the cores of T7
T8 and T9 for maximum "S"-meter reading . Do not be concerned at this point
if the recieve sensitivety is not too good . We still need to change the
Low Pass Filter on the transmitter section when we get to that stage .
You will also notice that USB and LSB have swapped around and dont seem to
be on frequency ... more on that in part 5
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*CB TO SIX METRES* Part5 - Other mods
By Shawn Baris ZR1EV
PO BOX 212
Brackenfell
7561
Republic of South Africa
*Anyone is free to use the following information for private use on the
provision that it is not used for commercial purposes . Permission is
granted by the author , ZR1EV , to publish and or distribute all or part of
the following on the condition that recognition is given to the author . In
the case of it being used in a newsletter or magazine , a copy of the
aforementioned should be sent to the above adress.*
It is not my intention to provide lengthy and detailed explinations on the
intricacies of SSB or bore those who know this subject well , but to point
out some of the basics with particular refference to this particular
chassis we are concerned with , and hopefully arm you with the neccissary
basic understanding of what we are doing . This will be invaluable when
fault finding needs to be undertaken at any stage .
I mentioned before that "USB and LSB have swapped around and dont seem to
be on frequency " . To understand what has happened here we have to have a
brief look at how SSB gets generated here . The "classic" method of
generating SSB involves a carrier generator spot on the IF frequency
feeding a balanced modulator of sorts . The resulting DSB signal is fed to
switched crystal filters : one for USB , one for LSB , possibly one for CW
and one for AM . As you can deduce from this , the manufacturing cost of
this approach would result in quite high prices on the finished product
being passed on to the consumer . This is probably one of the reasons that
most dedicated Amateur Transcievers have the CW filter fitted as an option,
rather than a standard fixture , in an attempt to keep the final cost to
the consumer low and by implication competitive .
The designers of this (and other rigs) have cleverly sidestepped these
problems with some ingeniuety . How AM is generated should be quite obvious
so we will not dwell on that now but rather just focus on SSB . Inspect the
innards of you rig . You will find only one large crystal filter located
roughly in the middle of the PCB . This filter is about 3-2.7 kc wide and
is a "LSB" filter centered round 10.6935 . (Some rigs are fitted with a
smaller rectangular filter , taking up about half the space . These are
"metal-oxide" filters , but for all intents and purposes can be treated the
same).
Suppose our carrier generator is set to 10.695 (standard 1st IF
frequency) . The 10.695 signal will pass through a balanced modulator chip,
IC4 (AN612) , where it is modulated by the audio signal in such a way that
the original 10.695 is balanced out and you are left with a double sideband
supressed carrier signal centered around 10.695 . This signal is now fed
into the crystal filter where the USB signal is heavily attenuated and the
LSB part of the signal is only slightly attenuated (filter insertion loss).
We are now left with a LSB SSB signal centred on 10.695 .
What if we want an USB signal ? The answer is to lower the carrier
frequency to below the filter passband : 10.692 is about right . The DSB
signal will now be generated centered around 10.692 , and after passing
through the Xtal filter , will only have the USB components remaining .
Note though that the USB signal will be centered around 10.692 and the LSB
and AM signals will be centered around 10.695 . Voila ! we generated both
USB and LSB with the same filter !
Now you can see that if the frequency from the synthesizer stays constant
between "mode" switching , The LSB and AM signals will appear on the band
"on freqency" , while the USB will be 3kc "off" . The designers got round
this "problem" by shifting the synthesizer output frequency by 3kc when the
USB mode is selected , thereby making all of this transparrent to the final
user , and all the "channels" RIT exactly at "12'o'clock irrespective of
the mode you have selected .
More to follow on this in Part6
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*CB TO SIX METRES* Part6 - More on mods
By Shawn Baris ZR1EV
PO BOX 212
Brackenfell
7560
Republic of South Africa
*Anyone is free to use the following information for private use on the
provision that it is not used for commercial purposes . Permission is
granted by the author , ZR1EV , to publish and or distribute all or part of
the following on the condition that recognition is given to the author . In
the case of it being used in a newsletter or magazine , a copy of the
aforementioned should be sent to the above adress.*
As probably expected , the story does not end there . If you look at the
circuit diagram , it becomes clear that both the frequency synthesizer and
the carrier oscillator gets pulled LOW on LSB , not the other way round as
implied in Part5 of this series . How could this be ?
The answer lies in the mechanics of mixing . On 27Mc an inversion of the
sidebands take place in the process of mixing the 10.695 DSB signal with
the output from the frequency synthesizer (coming in from the "high side"
of the wanted signal) . The result of the mixing process and the resulting
inversion of the sidebands means that one has to generate a LSB signal at
the IF frequency to realise an USB signal on 27Mc , Hence the "pulling" low
of both signals on both LSB and AM .
When we now convert the set to 50Mc , an additive mixing scheme is
employed , and the sidebands come out the "right way round" , with the
result that the frequency shifting scheme would have to be reversed . In
practice , this is quite a simple matter to accomplish . Some later
articles on conversions like these in the Amateur press , circa 1985/6 ,
would have you believe that it is simply a matter of redoing the labels on
the front panel . This will not work and you will have great difficulty in
getting the rig lined up to be on frequency on all the modes (if at all) .
Do the following and all will be well . You will see a grey wire running
from a point marked (T) next to Q4 in the synthesizer section to another
point also marked (T) on the pcb , next to Q18 . Desolder this wire from
the PCB where it enters the board next to Q18 . Pull it through toward the
mode switch . Next we need to look at the switch itself . There are two
sections to this switch . The one section applies full B+ to the final when
the rig is either in USB or LSB mode and routes the B+ via a hefty 6 ohm
wirewound variable resistor to the final when AM is selected (AM carrier
power adjustment) . Leave this as is , and locate the second section of the
switch which selects USB/LSB/AM . The wiring from this switch goes to
points (30), (19), (21) and (18) on the main PCB respectively .
Point (30) selects AM , point (19) selects USB , point (21) selects LSB
originally and point (18) is common . Swap the wires from points (19) and
(21) on the switch itself . Take two small silicon diodes (1N4148 or eq.)
and tie their cathodes together and solder the free end of the grey wire to
this junction . Shrinkwrap or slide an insulating sleeve over the exposed
sections of this connection . The two "free" ends of the two diodes
(anodes) solder directly onto the tags of the mode switch now connected to
point (19) and (30) respectively . For those who are not sure , the end of
the diode with the thickest colour band will be the cathode (apologies to
those who do know HI).
The modifications to the mode selector is now all done , and you can now
proceed to "net" the transciever . Adjust CT1 to net LSB and AM (one
adjustment for both modes) and CT2 for USB only . To enable the rig to
shift frequency on transmit as well as recieve , using the clarifier as a
RIT/TIT controll , remove D5 , and connect the unused end of the clarifier
controll to the emitter terminal of Q44 . Be sure to check the settings of
CT1 and CT2 after this mod as it will affect the 12'o'clock position on the
clarifier . I will discuss mods to the frequency selecting circuitry in
part7 ... see you then .
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*CB TO SIX METRES* Part7 - The channel selector logic
By Shawn Baris ZR1EV
PO BOX 212
Brackenfell
7561
Republic of South Africa
*Anyone is free to use the following information for private use on the
provision that it is not used for commercial purposes . Permission is
granted by the author , ZR1EV , to publish and or distribute all or part of
the following on the condition that recognition is given to the author . In
the case of it being used in a newsletter or magazine , a copy of the
aforementioned should be sent to the above adress.*
In my introduction to this series , in part1 , I mentioned the channel
selector and touched on some of the difficulties encountered when using
switches like these . This rig was designed for the American market and as
a result are originally fitted with a switch that selects frequency
according to the American 11M bandplan . This poses a few problems when any
modifications are attempted to change the operating frequency .
For example , you will notice that there are five spots between channels
where the frequency increments 20kc as the channel is incremented . Also ,
the frequency of channel 23 to 25 are not in sequence . Traditionally ,
mods involving a band change (say , to 10M) would require a piece of paper
with a list of channel numbers with corrosponding frequencies typed on it
attached to the rig , so one would be able to tell what frequency you are
operating on .
In my mods , I inserted an EPROM between the channel selector and the PLL
chip to "correct" all of these anomalies. It makes for a tidy, professional
"feel" to the mod and makes checking your frequency as simple as looking at
the LED readout . As the rig tunes in 10kc increments , one could program
the eprom so that Channel 1 (readout 01) will put you on 50.010 , channel
10 on 50.100 , and channel 40 on 50.400 etc. This is how to go about it :
The output from the channel selector switch will be a 6-bit binary "word"
that will be unique for each of the 40 channels . We will use each of these
unique 6-bit words to adress a specific location in an EPROM where you can
program the 8-bit code of your choice . The EPROM will output this 8-bit
word to the PLL chip and , voila ! you are on your frequency .
First you need to draw up a truth table . Make a list of all the channel
numbers and write down the binary from the channel selector down next to it
(you will find it listed in your service manual , or you can read the value
by measuring each bit directly off the PLL chip . The pins in question are
pin 15 (LSD) down to pin 10 (MSD) , in that sequence) . Next , convert the
binary words to HEX . All EPROM programmers I have seen work with HEX .
Select any channel above 27 (ch26-40 are all in sequence) . Read the full
9-bit binary off pins15-7 and write it down . Write down the frequency the
rig is operating on next to it . Click one channel up and do the same . The
binary should have changed by 1 .
Calculate all the binary codes for the range 50.010 up to 50.400 and when
that is done convert them all to HEX . You now have all the information you
need to program your EPROM . The eprom has an 8-bit output and the
synthesizer chip requires a 9-bit word . In practice you will find that the
state of the MSD pin stays the same throughout the 40 channels , with the
result that pin7 can be wired permanently to either logic1 or 0 .
I use a single 27C64 eprom , as there are a lot of them floating around
surplus . Remember to "pull down" the first six imputs with 1k resistors
and tie all the unused inputs to GND . I used a seperate 7805 3-terminal
voltage regulator IC to power te eprom and all these components can be
mounted on a small pcb or perfboard . I used short sections of ribbon cable
to carry the data from the switch and to the PLL chip . It is of course
possible to extend the basic 400kc tuning by programming another set of
codes for say,50.400 to 50.800 and then use one of the unused inputs to the
EPROM to switch "bands" this way . See you in Part8 where we will start the
mods to the transmitter .
----------------------------------------------------------------------
*CB TO SIX METRES* Part8 - The Transmitter section
By Shawn Baris ZR1EV
PO BOX 212
Brackenfell
7561
Republic of South Africa
*Anyone is free to use the following information for private use on the
provision that it is not used for commercial purposes . Permission is
granted by the author , ZR1EV , to publish and or distribute all or part of
the following on the condition that recognition is given to the author . In
the case of it being used in a newsletter or magazine , a copy of the
aforementioned should be sent to the above adress.*
Whenever any modifications or repairs are done to any piece of equipment ,
capable of radiating RF energy , it is of paramount importance that the
regulations concerning spurious and harmonic energy radiation are strictly
adhered to . This neccesitates the use of a spectrum analizer to verify
signal purity after these mods are done . I realize that a piece of test
equipment in that class is not standard equipment in most "shacks" , but
one would be able to most probably approach your local PMR company to check
the unit for you to ensure that your output is "clean" before you transmit
into your antenna .
The output from the frequency synthesizer is fed via a capacitive divider
circuit to pin4 of IC3 , where the IF injection frequency (injected into
pin1) is mixed with it to produce the 50Mc wanted output . There are what
appears to be two small chokes feeding pin6 and pin8 . L23 is indeed an RF
choke , but L3 is not .If you study the circuit carefully , it will become
clear that C37 and C38 are in series and connected across L3 to form a
parallel L-C Tuned circuit , which at the moment resonates at round 27Mc .
C37 and C38 also form a capacitive divider to do the impedance
transformation to the imput impedance of the cascode amplifier section of
IC3 (input on pin7) . L3 is wound on a carbon resistor of round 400 ohms to
lower the Q of this tuned circuit and thus improve stability and to
increase the effective bandwidth .
Initially , I used to just take a few turns off the coil , but I have
since decided to make a whole new coil instead and just replace the
existing L3 with the new one . I used a slightly thicker wire , and wound
exactly 15 turns onto a 470 ohm 0.5W carbon resistor . Cover the assembly
with heat-shrink tubing , and apply moderate heat till the shrink-tube
contracts and holds the windings in place . Remove C37 and C38 , and insert
47pF ceramic capacitor in C37 and a 27pF ceramic capacitor in C38 . I have
found that changeing L23 there was no percievable improvement , so , you
can leave L23 as is .
The output of the cascode buffer amplifier is fed into T4 and T5 which
forms a bandpass filter . Remove T4 and T5 and dismantle as described in
Part4 . T4 is a Mutsumi coil, part number 10CA006 and T5 is a Mitsumi ,
part number 10CB003 . Refer to part4 as to the orientation of the coils.
T4: Wind 1 turn onto the secon rib from the top starting at the leftmost
pin . Feed the wire down the gap provided on the former down to the centre
pin , form a pigtail , tin , and solder to the middle pin . Feed the wire
back up through the gap in the former and wind an additional 4 turns on the
topmost rib of the former . Feed the wire back down the gap , tin and
solder onto the righthand pin . Remove the internal capacitor , reassemble
and solder back into the PCB . Solder a 39pF miniature disk ceramic
capacitor across the primary (Ref.Part4)on the foil side of the PCB .
T5:Starting at the leftmost pin , wind 1 turn onto the second rib from the
top , crossover at the gap and continue to wind an additional 4 turns onto
the topmost rib , feed the wire down through the gap and tin , solder onto
the righthand pin . Remove the internal capacitor , replace the black
potcore , reassemble and solder back into the PCB . Add a 27 pF miniature
disk ceramic capacitor to the primary on the foil side of the PCB . All the
secondary windings can stay as is . Remove C42 and replace with a 2.2pF
ceramic capacitor . Remove C43 and replace with a 39pF . Remove C185 and
replace with a 18pf Ceramic disk capacitor .
We will continue with the modifications to the transmitter in part9
--------------------------------------------------------------------------
*CB TO SIX METRES * Part9 - The Transmitter section continued
*Anyone is free to use the following information for private use on the
provision that it is not used for commercial purposes . Permission is
granted by the author , ZR1EV , to publish and or distribute all or part of
the following on the condition that recognition is given to the author . In
the case of it being used in a newsletter or magazine , a copy of the
aforementioned should be sent to the above adress.*
We are now done with the "encapsulated" small coil formers and the rest of
the work can now be done without the aid of a magnifying glass (HI) .
Locate and remove T6 . This tuned interstage coupling transformer is wound
on a white plastic former and a lot of care should be excercised not to
overheat any of the pins , as the former may deform from the heat and
render it useless . The tuned primary of the transformer , can easily be
identified . The primary coil is wound from clear enamel coated copper wire
and the 1 turn secondary winding is wound using green enamel coated copper
wire .
Leaving the secondary winding as is , proceed to remove 2 turns from the
"top" of the winding , untill you are left with 5 turns on the former .
Form the free end of the wire with a small "longnose" pliers and feed it
through the hole in the base of the former where the original lead was fed
through . You will now notice thet the secondary and primary windings are
spaced too far apart (by the thickness of about two windings). "Pull" the
secondary winding lower down on the former (by pulling in the secondary
pins with a small pair of pliers) until the two windings touch . Trim the
pins to length and using a small hobby knife , scrape the insulation off
the pins and tin . Reinsert the transformer , solder back into the PCB and
replace C47 with a 27pF Disk ceramic capacitor .
Replace C49 with a 120pF disk ceramic capacitor . Replace L6 with a coil
consisting of 13 Turns wound onto a 1.5M ohm 0.5W carbon resistor . Replace
C52 with a 47pF 50V disc ceramic capacitor and C53 with a 120pF 50V disk
ceramic capacitor . Remove L7 and remove turns until you have 1.5 turns on
it . Prepare as with T6 and replace . Replace L9 with a coil made up as for
L6 . Modify L11 for 2.5 turns , L13 for 3.5 turns and L201 for 1.5 turns .
If you have difficulty locating L201 , you will find it (airwound coil)
between point (31) on the main chassis and the inner conductor of the
antenna socket . (some rigs have L201 and C201 omitted). Change C51 to 22nF
Replace C54 with an 82pF 100V disc ceramic capacitor , C55 with a 330pF
100V capacitor , C184 with a 68pF 50V disc ceramic capacitor and C201 with
an 18pF 100V disc ceramic capacitor . As "all coils are not wound equal",
You could experiment with slight changes in capacitor values for maximum
performance with minimum spurious output . C56 is fitted to some chassis ,
and can be removed in this mod . L13 is an air-spaced inductor with self
supporting windings , and can be streched or compressed for maximum output
When you notice that the peak tuning on T4 and T5 occurs when the slugs
are about the middle of the formers , you could INCREASE the capacitor
value across the primary by a few pF . The correct point for the cores are
about flush with the top of the screening cans . The same applies for the
Reciever coils , T8 and T9 with the exception of T7 . T7 should be tuned
for the loudest signal , not the loudest "hiss". This is important for a
good s/n ratio , so you can dig out those really weak signals on the band .
I have found that this occurs when the slug of T7 is a couple of turns from
the "bottom" of the coil former . If you cannot get this condition , try
changing the value of C100 by a couple of pF .
The rig should now "run up" smoothly . All of the rigs I have done so far,
exceeded the original specs on 27Mc on the reciever and practically always
got in excess of 10W PEP out of the transmitter (recomended PO) . If you
have a "lazy" 2SC1969 (Did a conversion once where I could do what I liked,
but could not get more than 8W PEP... Replacing the final did the trick),
You could try replacing the final with a 2SC1307 . These devices seem to
have more gain at 50Mc . See you in Part10 (final)
---------------------------------------------------------------------------
*CB TO SIX METRES* Part10 - The conclusion
By Shawn Baris ZR1EV
PO BOX 212
Brackenfell
7561
Republic of South Africa
The conversion I have described in Part1-9 has been in dayly use in my
shack for many years , and is the only rig I use on the six metre band . I
have done countless conversions like this for other enthusiasts and we have
all had hours of enjoyment out of them . The rig performs well on most of
the modes of propagation found on the six metre band , and with the
addition of a couple of extras , produces a stasion that you do not have to
apologise for .
I built up a push-pull linear amplifier using 2 x 6146B valves and my rig
drives them to max output easily . In addition I constructed a masthead
GaSFET preamplifier for that extra edge during weak openings . The antenna
will be one of your most important additions (as is the case with any
station , on any band) . It would be plain folly to expect "big" results
from a "small" antenna .
For CW enthusiasts , the addition of an 800Hz audio phase shift oscillator
feeding into the mic input would produce excellent results . I had a TONO
5000E connected to my rig for CW contacts with JA stations and had many
QSO's on CW with compliments recieved on the "tone".
The reciever is sensitive enough for serious meteor scatter work , and had
worked many stations on this mode . The rig lends itself well to mobile
operation and I used to often sit in the early morning traffic chatting to
the locals on 50.200 . You would be surprised at the excellent signals on
6M SSB when you are used to FM repeater operation .
Six metres is my favourite band and allways will be , and I hope you will
share in the excitement and thrill in homebrewing your own rig and using it
on your next DX contact . It was great fun for me and I hope it will be for
you too . Finally , an extract from my log book , using this rig,to wet
your appetite :
1991: FR5EL , JH6DFJ , TL8MB , JA6RJK , JK6PAC , JF6DEM , JE1BMJ , JR6WPT,
JA1VOK , JR1VSP , CE8ABF , LU7VB , JF1IRW , JR6WPT , JS6CDB , JF1IRW
JI1CQA , ZS6WB , FC1JG , LU8MBL , PY5CC , LU8AHW , LU6DLB , LU3DCA ,
LU8AJK , LU9AEA , ZP6XDW , LU1DMA , LU3EX , LU8DIO , ZP5ZR , CX8BE ,
OE4WHG , IK2GSO , LU7MEC , CX4HS , FC1GTU , IK8MKK , I0AMU , FC1JG ,
9H1BT , 9H5EE , SV1EN , OE6LOG , YU3ZV , SV1DH , CN8ST , ZS9H ,
V51KC . etc.
1992: I7CSB , F6CER , 9H1GB , I4CIL , I4SJZ , IK4EWN , FC1JG , IK8DYD ,
G3KOX , G3VYF , GD3AHV , G3WOS , G0LCS , GW3LDH , GW8ZCP , G1ITE ,
G4ICO , G3OIL , G4GAI , DL7AV , G3APY , GW4LXO , CT3FT , SV1DH ,
SV1EN , 9H5EE , 9H1PA , 9H1CG , 9H1GB , 9H5BW , 9H5AZ , FC1JG ,
EA3VHF(BEACON) , LU8MBL , LU7MEC , LU9MA , JR6WPT , YU3ZV , JN1BPM ,
JH1ECU , ZR5ADQ , ZS5DW , etc etc .
Happy mods and I will be on the lookout for you the next time the band
opens !
vy 73 de Shawn ZR1EV JF96ic
SMIRK#5673 (Six Metre International Radio Klub)
-----------------------------------------------------------------------------
Applicable Copyright information:
*Anyone is free to use the following information for private use on the
provision that it is not used for commercial purposes . Permission is
granted by the author , ZR1EV , to publish and or distribute all or part of
the following on the condition that recognition is given to the author . In
the case of it being used in a newsletter or magazine , a copy of the
aforementioned should be sent to the above adress.*
-----------------------------------------------------------------------------

NOTE: only licensed Amateur Radio operators can operate this modified equipment on assigned Amateur frequencies/bands.